1. Field of the Invention
The present invention relates to a fixing device for fixing a toner image by applying heat and melting a toner image onto a surface of a printing medium, a control device for controlling the fixing device, and an image forming apparatus having the fixing device
2. Discussion of the Background Art
In an image forming apparatus, such as a printer, a copier, a facsimile, etc., it is well known that a toner image is fixed onto a printing medium, such as a sheet, etc., by applying heat and melting the toner image. In general, in a fixing device, power is supplied to a heater serving as a heat source so that the heater heats a fixing member, such as fixing roller, a fixing belt, etc., and thereby fixing the toner image onto the printing medium.
In such a fixing device, to maintain a fixing temperature at a prescribed level during the fixing, the heater is supplied with power and increases its temperature to a prescribed level (i.e., a target control temperature), so as to get ready for the fixing. A printing medium is then conveyed through the fixing device maintaining the fixing temperature.
Such power drive of the heater (i.e. heating control) is executed by controlling power distribution thereto. To control the power distribution to the heater, temperature is generally detected by a temperature sensor, such as a thermistor (TM), etc., arranged in the fixing device, and a power distribution time period (i.e. Duty) per hour (i.e. a control cycle) is changed in accordance with a difference between detected and target control temperatures.
As a method of calculating the Duty, feedback control, such as PID control, etc., is frequently used as mentioned below.
Specifically, PID control is executed by combining Proportional (P), Integral (I), and differential (D) processing while optimizing plural parameters in accordance with a difference between target and current values.
When fixing temperature is controlled, Duty is calculated using the formula 1 per a control cycle, wherein Kp represents proportional gain, TI represents an integral time period, TD represents a differential time period, e(t) represents a difference between temperatures of a control target and a fixing member (=r(t)−y(t)), r(t) represents target control temperature, y(t) represents temperature of a fixing member, and T represents a control cycle.DUTY=Kp(e(t)+1/TI×∫i0e(t)dt+TD×de(t)/dt).  Formula 1;
Since the duty is calculated based on temperature information using a PID controller, fixing temperature can be stable at around the target level as shown in FIG. 10. Specific aspect of controlling power distribution when a control cycle is 200 ms and the duty calculated by the PID controller is 30% is illustrated in FIG. 11. As shown, the power distribution is started from a zero cross point of a commercial power supply and completes at 30% of the 200 ms, specifically, when 60 ms has elapsed. Such a control manner is called an on/off system.
A lot of inrush current flows when heater starts turning on in the on/off system when a halogen heater is employed. Thus, when a commercial power supply is connected to the halogen heater, a voltage of a commercial power supply line changes, and a flicker phenomenon that adversely affects instruments connected to the power supply line occurs.
To avoid this phenomenon, a phase change control system is widely spreading as a soft start system in that a time period of power distribution to the heater gradually increases when the power distribution starts as shown in FIG. 12. In a certain condition, a soft stop system is used together for the same reason to gradually decrease the power distribution time period at the time of turning off as shown in FIG. 13.
An aspect of controlling power distribution using both of the soft start and stop systems on condition that a control cycle is 200 ms and a duty is 30% is illustrated in FIG. 14. With the above-mentioned soft start and stop systems, the flicker phenomenon can be avoided due to reduction of sharp change of the voltage. However, since a higher harmonic wave current is induced due to execution of the phase control, the soft start and stop systems need be executed by balancing countermeasures against the flicker and high harmonic wave current.
Further, the most of fixing devices employ plural heater elements to deal with various widths of sheets conveyed there through these days. In such a situation, the above-mentioned soft start and stop systems are employed in each of the plural heaters, for example two heaters as shown in FIG. 15.
However, as shown, when the plural heaters are turned on, soft start and stop time periods for these overlap each other, the higher harmonic wave current further increases as a problem.
Then, as discussed in the Japanese Patent Applications 10-186940, 6-236128, 9-311584, and 59-111669, a turning on start time for a heater is deviated from that for other's not to overlap the soft start and stop time periods with each other to effectively suppress a change of voltage and occurrence of high harmonic wave current.
However, since the duty changes at real time in accordance with temperature of a fixing member as mentioned earlier, the soft start and stop time zone sometime overlap with other's depending on a value of the duty as a problem, when the duty calculation is executed by the PID controller and each of the heater power distribution start times is simply shifted.